PHP Warning When No Access Plan

### Reproduction Steps

1. Ensure WP_DEBUG and WP_DEBUG_DISPLAY are on
2. Delete all access plans for a Course and save
3. Refresh the page
4. Start adding a new access plan and close the popup

### Expected Behavior

No warning

### Actual Behavior

```
Warning: Undefined variable $checkout_url 
```

in includes/admin/views/access-plans/access-plan.php line 99